A standard four-high rolling stand of the type used for rolling strip steel has a pair of vertically spaced nip-defining working rolls of relatively small diameter. Respective upper and lower backup rolls of larger diameter bear respectively down and up toward the nip upon the upper and lower working rolls, respectively. Thus the bendability of the small-diameter working rolls is largely canceled out by the rigidity of the larger-diameter backup rolls so that the large forces required for rolling can be brought to bear on the workpiece.
To achieve even greater uniformity in workpiece thickness, it is now standard to provide six-high roll stands. Such stands have inner and outer backup rolls, the latter being of greater diameter than the former. Even with such a system the workpiece is normally subjected to somewhat more pressure at its longitudinal edges than at its center so that the working rolls wear more at their portions corresponding to the edges of the strip being rolled.
German Pat. No. 955,131 filed by T. Sendzimir with a claim to a U.S. priority date of Feb. 25, 1943 suggests that this problem can be somewhat alleviated by axially oppositely offsetting the two inner backup rolls so that one end of the one working roll and the opposite end of the other working roll are not backed up. A similar arrangement is described in U.S. Pat. No. 3,857,268. This allows some deformation of the working rolls in the corresponding regions and thereby ensures more uniformity of workpiece thickness. Such axial offsetting of the inner backup rolls allows the stand to be adjusted for the particular workpiece width exactly to produce a rolled product of relatively uniform thickness. In order further to prevent the workpiece from being left slightly too thick at its edges, the working rolls in this system are somewhat crowned, that is are of somewhat greater diameter at their centers than at their ends.
It is also known from the U.S. Pat. No. 4,194,382 to T. Kajiwara to provide the axially displaceable inner backup rolls with bending units. In addition in this system, the effective length of the rolls decreases outward from the nip, with the inner backup rolls having shorter effective lengths than the working rolls and the outer backup rolls being shorter than the inner ones.
German patent documents 2,334,492 and 3,115,461 have axially displaceable inner backup rolls and bendable working rolls that are operated by a controller in such a manner as to produce a perfectly flat workpiece.
Another such system is described in copending patent application Ser. No. 352,520 filed Feb. 26, 1982 by Hans Rommen et al and in copending patent application Ser. No. 558,187 filed Dec. 5, 1983. These systems employ a six-high roll stand that has a frame, upper and lower working rolls journaled in the frame for rotation about respective parallel upper and lower working-roll axes and defining a workpiece nip, respective upper and lower backup-roll guide elements vertically flanking the working rolls and each at least partially vertically displaceable relative to the frame, and respective upper and lower backup-roll journal blocks axially displaceable but vertically fixed relative to the guide elements and defining parallel upper and lower inner axes flanking the working axes. Respective upper and lower working-roll guide elements level with the working rolls are each at least partially vertically displaceable relative to the frame. Respective upper and lower working-roll journal blocks axially displaceable but vertically fixed relative to the respective guide elements define the working axes. Respective vertically effective actuators exert a force at least generally parallel to the planes between each working-roll journal block and the frame to bend the working rolls and to displace the force axially relative to the working-roll journal blocks. Respective inner backup rolls are journaled in the blocks for rotation about the inner axes and bearing radially toward the nip on the working rolls. The inner backup rolls are axially displaceable in the guide elements. A pair of outer backup rolls are journaled in the frame for rotation about outer axes coplanar with the respective inner axes and bearing radially toward the nip on the inner backup rolls. A control unit including respective vertically effective actuators exerts a force at least generally parallel to the planes between each journal block and the frame to bend the inner backup rolls and for displacing the force axially relative to the journal blocks. A strip workpiece is passed according to this earlier invention in a travel direction through the nip and its thickness is detected downstream of the stand both at its center and edges. The bend of the inner backup rolls is varied when the thickness of the workpiece at its center varies from a predetermined range to return the center thickness to the range and the bend of the working rolls is varied when the thickness of the workpiece at its edges varies from a predetermined range to return the edge thickness to the respective range.
Thus, in addition to varying the bend of the inner backup rolls to correct the central workpiece thickness, the bend of the working rolls is oppositely varied but to a much smaller extent, and in addition to varying the bend of the working rolls to correct the edge workpiece thickness, the bend of the inner backup rolls is oppositely varied but to a much smaller extent.
The workpiece thickness is detected by measuring workpiece tension downstream of the nip. This can be done according to the invention by deflecting the workpiece downstream of the nip over a tension-measuring roller.